An apparatus which can identify which of a plurality of types of plugs has been inserted in an earphone jack has been known.
For example, a portable telephone including an earphone jack for five electrodes commonly used for a four-electrode plug and a five-electrode plug has been known. The four-electrode plug transmits a microphone signal, a left audio signal, a right audio signal, and a ground voltage, and the five-electrode plug transmits a PTT switch signal, a microphone signal, a left audio signal, a right audio signal, and a ground voltage.
When a plug is inserted, a tone signal is output from a third jack terminal. When a four-electrode plug is inserted here, the tone signal is output from a first jack terminal through a four-electrode earphone as a leakage signal, which is input to a control circuit after it is amplified. When a five-electrode plug is inserted, no leakage signal is output. According to such a configuration, which of the four-electrode plug and the five-electrode plug has been inserted into the plug can be identified.